On the Great Barrier Reef, you can dive year-round, although the best conditions for diving are from late August to early December. For liveaboard diving, the best time to go is July through November. In the Coral Sea, the visibility is best in June and in September. If you want to see the Minke whales, they're seen mostly from June to August. From April through June, the whale sharks can be seen near Ningaloo, in Western Australia. Australia Tourism has an excellent reference on dive locations and regions here .